Depth operations

ABSTRACT

Described are a video graphics system, graphics processor, and methods for rendering three-dimensional objects. A buffer is partitioned into tiles of pixels. Each pixel of each tile includes at least one sample. A primitive is received and determined to fully cover one of the tiles of the buffer. A section of the primitive that maps to the fully covered tile is tested to determine whether that section of the primitive may be drawn in its entirety. A value is stored in the buffer for the fully covered tile in response to determining that the section of the primitive may be drawn in its entirety. The value indicating that every sample of the fully covered tile has a depth value determined by the primitive.

FIELD OF THE INVENTION

The invention relates generally to graphics processing. Moreparticularly, the invention relates to systems and methods ofaccelerating three-dimensional graphics processing using hierarchical Zmechanisms.

BACKGROUND

For displaying three-dimensional representations of objects on a displayscreen, graphics-processing systems typically partition each object intogeometric primitives. Geometric primitives are elementary graphicsbuilding blocks, examples of which include points, lines, images,bitmaps, vectors, and polygons. Triangles are a commonly employed formof primitives. Each primitive is stored as a set of vertices, with eachvertex having associated display parameters, including color parameters,display (or pixel) location parameters, and texture parameters. Eachprimitive passes through a graphics pipeline. A rasterizer identifiesthe pixels corresponding to each primitive.

Three-dimensional scenes can have numerous objects, with many of theobjects having thousands of primitives; thus, the rendering process isgenerally computationally intensive and complex. Often, a 3-D scene hasoverlapping objects. Depending upon the extent of overlap, opaqueobjects in the foreground can occlude a portion of or all of an objectin the background. Thus, such objects may be invisible. Accordingly, animportant part of the 3D rendering process is the use of depth (i.e., Z)information.

Some graphics-processing systems determine which objects are visible ata late stage in the graphics pipeline, often after determining textureand color information for the pixels. The graphics processor thendiscards the pixel information for the occluded object. Computing pixelinformation for an invisible object, therefore, is an inefficient use ofprocessing power and unnecessarily prolongs the rendering process.

As a mechanism to reduce unnecessary computations during the 3Drendering process, graphics processing systems employ a hierarchicalZ-buffering technique before rendering a pixel. The hierarchicalZ-buffering technique (referred to herein as Hi-Z) entails comparing theZ values of pixels at the same (x, y) display location with a stored Zvalue for that location. The stored Z value represents the pixelpresently deemed closest to the viewer. Pixels with a lesser Z valuethan the stored Z value are occluded, and are not rendered. A new pixelwith a Z value that is higher than the stored Z value, however, becomesthe closest (i.e., visible) pixel and is therefore rendered. Inaddition, and update to the Hi-Z buffer stores this new Z value toreflect the new closest pixel.

To reduce the computational load to determine possible occlusions priorto rendering, Hi-Z determinations can operate on tiles (i.e., a matrixof multiple pixels). Early Hi-Z techniques stored a far Z value for eachtile. For a given incoming geometry, it could then be determined whetherthat a near-Z value of the geometry will fail a depth test when comparedto the stored far Z value of the tile. The comparison served primarilyto reject (i.e., cull) pixels that would not be drawn, thereby avoidingthe inefficient process of rendering a pixel only to be later discarded.

Another means by which a pixel may be occluded is through stenciling.Similar to background pixels occluded by foreground objects, renderingpixels occluded by a stencil is wasteful. In one typical use, a stencilis a mask placed on the output scene that produces a shadow inaccordance with the location of a light source. The stencil occludespixels at the same x, y location with a smaller Z value than thestencil, i.e., it falls within the shadow. A pixel thus covered by thestencil is invisible (i.e., hidden in the shadow) and is not rendered.Shadow volumes are but one of many different exemplary uses for stencil.As an enhancement to the process of identifying stencil-occluded pixels,graphics processing systems can employ a hierarchical stencil techniquethat, like Hi-Z, operates on tiles of pixels. Hi-S can provide an earlyindication of whether the tile will pass (be rendered), will fail (beculled), or whether the outcome is unknown.

SUMMARY

In one aspect, the invention features a method of renderingthree-dimensional objects. The method comprises partitioning a bufferinto tiles. Each tile includes a plurality of pixels and each pixel ofeach tile includes at least one sample. A primitive, associated with anobject, is received. That the primitive fully covers one of the tiles ofthe buffer is determined. A section of the primitive that maps to thefully covered tile is tested to determine whether that section of theprimitive may be drawn in its entirety. A value is stored in the bufferfor the fully covered tile in response to determining that the sectionof the primitive may be drawn in its entirety. The value indicates thatevery sample of the fully covered tile has a depth (Z) value determinedby the primitive.

In another aspect, the invention features a graphics processorcomprising a tile scan converter receiving a primitive associated withan object to be processed for drawing and identifying a tilecorresponding to the primitive. The tile includes a plurality of pixels,each pixel including at least one sample. A hierarchical Z test moduledetermines that the primitive fully covers the tile and that a sectionof the primitive that maps to the fully covered tile may be drawn in itsentirety. A buffer, in communication with the hierarchical Z module,stores a value in response to a determination by the hierarchical Z testmodule that the section of the primitive may be drawn in its entirety.The value indicates that every sample of the fully covered tile has adepth (Z) value determined by the primitive.

In still another aspect, the invention features a video graphics systemcomprising a graphics processor with a tile scan converter receiving aprimitive associated with an object to be processed and identifying atile corresponding to the primitive. The tile includes a plurality ofpixels. Each pixel includes at least one sample. A hierarchical Z testmodule determines that the primitive fully covers the tile and that asection of the primitive that maps to the fully covered tile may bedrawn in its entirety. A buffer, in communication with the hierarchicalZ module, stores a value in response to a determination by thehierarchical Z test module that the section of the primitive may bedrawn in its entirety. The value indicates that every sample of thefully covered tile has a depth (Z) value determined by the primitive.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantages of this invention may be betterunderstood by referring to the following description in conjunction withthe accompanying drawings, in which like numerals indicate likestructural elements and features in various figures. The drawings arenot necessarily to scale, emphasis instead being placed uponillustrating the principles of the invention.

FIG. 1 is an embodiment of a graphics-processing system constructed inaccordance with the invention, the graphics-processing system includinga graphics processor and a display screen.

FIG. 2 is a functional block diagram representation of a portion of anembodiment of the graphics processor in accordance with the invention.

FIG. 3A is a block diagram of an embodiment of an exemplary tilerepresentation as stored in buffer memory, the tile being comprised ofan 8×8 matrix of pixels and represented by a single z-plane, each pixelof the tile comprising one sample.

FIG. 3B is a block diagram of an embodiment of an exemplary tilerepresentation as stored in buffer memory, the tile being comprised ofan 8×8 matrix of pixels and represented by two planes, each pixelcomprised of one sample.

FIG. 3C is a block diagram of an embodiment of an exemplary tilerepresentation as stored in buffer memory, the tile being comprised ofan 8×8 matrix of pixels and represented by two planes, each pixelcomprised of two samples.

FIG. 4 is a flow diagram representation of an embodiment of a processfor accelerating 3D rendering using hierarchical Z buffering inaccordance with the invention.

FIG. 5 is a flow diagram representation of an embodiment of a processfor compressing stencil values of a tile into a single stencil value inaccordance with the invention.

FIG. 6A is a block diagram of an embodiment of an exemplary tilerepresentation as stored in buffer memory for a tile comprised of an 8×8matrix of pixels, each having eight samples, and represented by onez-plane.

FIG. 6B is a block diagram of the exemplary tile representation of FIG.6B after stencil value compression.

FIG. 7A and FIG. 7B are a flow diagram representation of an embodimentof a process for performing fast stencil operations in accordance withthe invention.

DETAILED DESCRIPTION

Graphics-processing systems can employ various mechanisms, describedherein, to accelerate the processing rate of depth (Z) and stenciloperations, such as those used in stencil shadow values, in Z-onlypre-passes, in shadow-depth map generation, and in various applicationsof computation masking. One such mechanism, referred to as “Fast Z”,bypasses detail rasterization for a tile of pixels if a hierarchical Ztest (herein called Hi-Z) determines that a primitive fully covers thetile and that the tile passes the Hi-Z test (i.e., the old Z data willbe overwritten). In addition, the Fast Z mechanism can employ acompression mechanism for storing a representation of the Z data of eachsample of the tile.

Another mechanism, called “Fast Stencil”, can also be used to bypassdetail rasterization for a tile of pixels. Fast Stencil operationsperform a hierarchical stencil test (herein called Hi-S) to determinewhether a primitive fully covers the tile and whether all samples of thetile will undergo the same stencil operation. If either of two otherconditions is also satisfied, then an update to the stencil values inthe frame buffer for the tile can be performed without detailrasterization. These conditions are (1) the old stencil values are to beentirely overwritten, or (2) a single stencil value represents thestencil values for all samples of the tile. Fast Stencil operationscapitalize on a stencil compression technique, described herein, thatdetermines when every sample of a given tile have the same stencil valueand consequently reduces these stencil values to a single stencil value.

FIG. 1 shows an embodiment of a graphics processing system 10 includinga computing system 12 in communication with a display screen 14. Thecomputing system 12 includes a graphics-processing unit 16, a centralprocessing unit (CPU) 18, and memory 20, each connected to a system bus22. Various examples of computing systems within which the presentinvention may be embodied include, but are not limited to, personalcomputers (PC), Macintosh computers, workstations, laptop computers,server systems, hand-held devices, and game consoles.

The memory 20 includes non-volatile computer storage media, such asread-only memory (ROM), and volatile computer storage media, such asrandom-access memory (RAM). Within the RAM are program code and data(i.e., graphics information). Program code includes, but is not limitedto, application programs, a graphics library, and an operating system(e.g., Windows 95™, Windows 98™, Windows NT 4.0, Windows XP™, Windows2000™, Vista™, Linux™, SunOS™, and MAC OS™). Examples of applicationprograms include, but are not limited to, standalone and networked videogames, simulation programs, word processing programs, and spreadsheetprograms.

The display screen 14 can be any device suitable for displayingcomputer-graphics generated images. Examples of display screens include,but are not limited to, computer monitors, television monitors, liquidcrystal displays (LCD), and cathode ray tube displays. In the exampleshown, a three-dimensional image 24 comprised of a spherical object 26and planar objects 28-1, 28-2 (generally, 28) appears on the displayscreen 14. A light source 30 disposed on one side of the sphericalobject 26 causes the object 26 to cast a shadow 32 onto the planarobjects 28. Accordingly, the image 24 is an example of a display inwhich portions of objects are not visible because of shadow or becauseof an opaque object in its foreground.

The display screen 14 is partitioned into a plurality of regions ortiles 34. Each tile 34 corresponds to a matrix of display pixels 36. Toincrease computing efficiency, certain graphics computations can operateon tiles rather than on individual pixels or samples. In the exampleshown, each tile 34 is an 8×8 matrix of pixels. Tiles 34 can have othersizes (e.g., 4×4, 4×8, and 8×4) without departing from the principles ofthe invention.

In brief overview, the graphics-processing unit 16 receives graphicscommands from the CPU 18 and graphics information from the memory 20.Based on the received commands and graphics information, thegraphics-processing unit 16 generates the image 24 that appears on thedisplay screen 14. Devices other than the memory 20 can operate as inputsources of graphics information. The present invention accelerates the3D rendering process by identifying tiles of pixels that can bypassdetail rasterization.

FIG. 2 shows an embodiment of the graphics-processing unit 16 includinga primitive assembly 50, a tile scan converter 54, and a hierarchical Z(Hi-Z) and stencil test module 58 in communication with a Hi-Z andstencil cache 62 and with a frame buffer 66. In general, the framebuffer 66 is video memory allocated to hold the graphics information forthe pixels from which the video display (i.e., frame) is drawn. Inaddition, each pixel of the frame buffer 66 is comprised of one or moresamples (i.e., sub-pixels).

Each tile 34 of the display screen 14 maps to a tile of pixels (andsamples) in the frame buffer 66. The graphics information for eachsample of each pixel to be drawn on the display screen 14 includes acolor value, a depth (Z) value, and a stencil value. Instead ofincorporation within the frame buffer 66, a separate Z-buffer forholding Z values and a separate stencil buffer for holding stencilvalues may be used without departing from the principles of theinvention.

The graphics-processing unit 16 also includes a detail scan converter70, a shader 74, and an optional late Z and stencil test module 78. Inone embodiment, an application-specific integrated circuit (ASIC) chipor chipset embodies the graphics-processing unit 16.

Each of the various modules (e.g., 58, 62, 66) of the graphicsprocessing unit 16 includes processor logic, which may be comprised ofcircuitry, software, or a combination thereof, for executing theparticular functionality of that module. In FIG. 2, such processor logicis generally denoted as reference numeral 68.

During operation, the primitive assembly 50 receives vertices ofgraphics information and from such vertices produces graphics primitives(e.g., triangles). From primitives received from the primitive assembly50, the tile scan converter 54 identifies each tile fully or partiallycovered by a given primitive. Associated with each tile is a set of (x,y) coordinate locations (corresponding to the pixels and samples in thetile) and a z-plane equation representative of the plane within whichthe primitive lies. In one embodiment, the z-plane equation includes apoint, i.e., an (x, y) coordinate location within the plane of theprimitive, and a pair of slopes, dx/dz and dy/dz. The z-plane equationenables derivation of a Z value for each sample of the tile.

The tile scan converter 54 sends the (x, y) coordinate locations andz-plane information of the tile to the Hi-Z and Hi-S test module 58. TheHi-Z and Hi-S test module 58 determines whether the tile is fullycovered by the primitive (i.e., every sample of the tile is within thez-plane representing the primitive).

Associated with each tile are a minimum Z value and a maximum Z value,each derived from the smallest depth value and the largest depth value,respectively, of all samples in the tile. Together, the minimum Z andmaximum Z values specify a range of Z values within a particular tile.

The Hi-Z and Hi-S memory 62 stores tiles (referred to as H-tiles). Thereis a one-to-one correspondence between the H-tiles and tiles of theframe buffer 62. Each H-tile has a minimum Z value, a maximum Z value, astencil value range, and a stencil flag, which together provide acompressed representation of the Z and stencil values stored in theframe buffer 66 for the corresponding tile. This compressedrepresentation enables a coarse testing of the corresponding tile.

The Hi-Z and Hi-S test module 58 communicates with the Hi-Z and stencilmemory 62 to retrieve the hierarchical Z value range, stencil valuerange, stencil flag, or combinations thereof, for the H-tilecorresponding to an incoming tile under test. With the retrievedhierarchical Z value range and stencil information (i.e., stencil valuerange and flag), the Hi-Z and Hi-S test module 58 determines whether thetile being tested will fail (will not be drawn), will pass (a likelihoodto be drawn), or if the outcome is unknown (not determinable). The basisfor passing or failing a given tile can be on the stencil test alone, onthe Z test alone, or on a combination of the two. Techniques forcomparing an incoming tile with its corresponding H-tile to determinewhether that tile passes, fails, or is indeterminable are described inSeiler et al., U.S. application Ser. No. 10/790,953, titled “Method andApparatus for Hierarchical Z Buffering and Stenciling”, the entirety ofwhich application is incorporated by reference herein. Some embodimentsof graphics-processing systems perform such testing on “quad tiles”,i.e., a 4×4 quad tile for an 8×8 tile, and operate on four such quadtiles in parallel.

If the primitive fully covers the tested tile and the outcome of thetest is that the tile will pass, the processing of tiles can proceeddirectly from the Hi-Z and Hi-S test module 58 to the writing of Z andstencil values to the frame buffer 66. In effect, the processingbypasses the detail rasterization associated with the detail scanconverter 70, shader 74, and test module 78 branch of the graphicspipeline. By bypassing the detail rasterization, the graphics operationscan take place at the tile level (i.e., work with units of tiles),rather than at the pixel or sample levels (as is done at the shader 74and z/stencil test module 78).

If the primitive does not fully cover the tile, or if the outcome of thetest is unknown or that the tile will fail, the processing of the tileproceeds to the detail scan converter 70. The detail scan converter 70determines those pixels of a tile that are fully or partially covered bythe primitive, and produces a sample mask for each fully or partiallycovered pixel. In general, the shader 74 includes program code fordetermining a final color and z-value for a pixel, often adding complexshading effects, e.g., texture mapping and transparency, to theappearance of an object. To implement shading effects, the shader 74 maycommunicate with a texture block in order to add a texture to the pixelsbeing rendered. An optional late Z and stencil test module 78 can beused to test the visibility of pixels.

Graphics information for the pixels continuously passes to the framebuffer 66, e.g., from the late depth and stencil test module or from theHi-Z and Hi-S test module 58, until the frame buffer 66 receives a fullframe of graphics information. At that time, the frame buffer 66 mayprovide the completed frame to an output device, such as the displayscreen 14.

FIGS. 3A-3D show exemplary representations of a tile, comprised of an8×8 matrix of pixels, as stored in the frame buffer 66. In general, eachtile representation includes a stencil value (e.g., 8-bit) for eachsample in the tile and a z-plane value (e.g., 96 bit) for each primitivecovering at least one sample in the tile. For tile representationshaving more than one plane, a mask comprised of one or more bits (hereincalled Pmask) associates each sample to one of the planes. For example,a single bit per sample can be used to distinguish between two z-planes,two bits per sample for distinguishing among as many as 4 z-planes, andthree bits per sample for distinguishing among as many as 8 z-planes.The Z data for a given sample can be derived from the z-plane to whichthat sample points. As described in more detail below, a single stencilvalue, a single z-plane value, or both can be used to represent thestencil values and Z values for all samples in a tile.

FIG. 3A shows a particular exemplary representation of an 8×8 pixel tilefor which each pixel has one sample (S1). Each sample has an 8-bitstencil value (the size of the stencil value being exemplary).Accordingly, 512 bits (64 samples*8-bits) of frame buffer memory holdstencil values for this particular tile. In addition, all samples areassociated with the same z-plane (z-plane 0), which is a condition thatoccurs when a primitive fully covers a tile that is then written to theframe buffer, as described herein in connection with FIG. 4. Here, forexample, 96 bits hold the pointer to the z-plane 0. Because all samplesrefer to the same z-plane, no PMASK is needed to identify the particularz-plane to which each sample points.

FIG. 3B shows an exemplary representation of an 8×8 pixel tilerepresentation similar to that shown in FIG. 3A, except that in FIG. 3Bsamples (S1) correspond to two different z-planes. The tilerepresentation includes pointers to each z-plane (here, z-p1 and z-plane0), each stored as a 96-bit value. A 64-bit PMASK, 1 bit for each of the64 samples, identifies to which of the two z-planes each samplecorresponds.

FIG. 3C shows an embodiment of an 8×8 pixel tile representation similarto that shown in FIG. 3B, except that in FIG. 3C each pixel has twosamples (S1, S2). Accordingly, 1024 bits (128 samples*8-bits) of theframe buffer memory 66 hold stencil values, and a 128-bit PMASK, 1 bitfor each of the 128 samples, identifies to which of the two z-planeseach sample corresponds.

With an increase in the number of samples per pixel, there is acorresponding increase in frame buffer memory for storing the stencilvalues associated with the samples. For examples, for eight samples perpixel, to hold stencil values for each sample requires 4096 bits (64pixels*8 samples/pixels*8-bits/sample). A stencil compression technique,described below in connection with FIG. 5 and FIG. 6, can reduce theamount of memory (and improve bandwidth) needed to represent the stencilvalues of all samples in a tile.

Fast Z Operation

FIG. 4 shows an embodiment of a Fast Z process 120 for acceleratingdepth (Z) processing in accordance with the invention. Performance ofthe Fast Z process 120 may or may not occur in conjunction with a Faststencil process, described below. In the description of the process 120,reference is also made to FIG. 2. To illustrate that thegraphics-processing system can implement the Fast Z process 120 withoutimplementing stencil operations, the description refers to the Hi-Z andHi-S test module 58 as Hi-Z test module 58′ and to the Hi-Z and Hi-Smemory 62 as Hi-Z memory 62′.

At step 124, the tile scan converter 54 receives a primitive from theprimitive assembly 50. The tile scan converter 54 scans through thetiles to identify (step 128) each tile covered fully by the primitive(i.e., no portion of the tile falls outside the perimeter of theprimitive). At step 132, the Hi-Z test module 58′ compares a fullycovered tile with its corresponding H-tile to determine if the tile willpass, will fail, or if the outcome is unknown, as described above.

If the Hi-Z test module 58′ determines (step 136) that the fully coveredtile will pass, this indicates that the old Z data for the tile withinthe frame buffer 66 can be replaced. At step 140, processor logic (i.e.,circuitry, software, or both) replaces the old Z data by writing asingle z-plane value to the frame buffer 66 for the tile, i.e., thez-plane corresponding to the plane of the primitive that fully coveredthe tile. Writing a single z-plane value to the frame buffer 66 can havethe memory-saving effect of reducing a list of multiple z-planes to asingle z-plane. In addition, the writing and reading of the singlez-plane value can take place within a single clock cycle. Further, theprocessor logic may update the graphics information (i.e., depth values,and/or stencil values) for each pixel sample of the tile with the valuesdescribed by the single z-plane.

This updating of the frame buffer bypasses detail rasterization (FIG.2), and thereby accelerates the generation of the complete frame. Inaddition, the Hi-Z test module 58′ updates (step 144) the minimum Z andmaximum Z data in the corresponding H-tile within the Hi-Z memory 62′ toreflect the Z data of the fully covered tile and to indicate that thecorresponding H-tile has only one z-plane. If instead, the Hi-Z testmodule 58′ determines that the tile will fail or if the outcome isunknown, the tile undergoes (step 148) detail rasterization.

Fast Z operations can be used to perform “Fast Clear” operations. Inbrief overview, a graphics-processing system can use fast clearoperations to clear or reset the data stored in the frame buffer to adefault Z and default stencil values. To effect a Fast Clear operation,the graphics-processing system provides a primitive that fully coversall tiles in the frame buffer. The design of this primitive ensures itto be “visible” (i.e., to pass Hi-Z and Hi-S tests). Accordingly, whenprocessing this primitive, the graphics-processing system determinesthat the primitive covers every tile fully and, after bypassing detailrasterization, overwrites each tile in the buffer with a single z-planeequation and single stencil value. Although described as a mechanism forclearing Z data and stencil values concurrently, Fast Clear operationscan be adapted to perform a clear of Z data without affecting stencilvalues or a clear of stencil values without affecting Z data.

Stencil Value Compression

In many 3-D scenes, shadows obscure but a minor portion of the scene.Accordingly, large sections of a scene are not in shadow, and for thosetiles corresponding to those sections, the stencil values of the samplescan show little if any variation. Even for those tiles corresponding tosections that are in shadow, every sample within a given tile can oftenhave the same stencil value. This principle is generally applicable alsofor non-shadow uses of stencil. For one, stencil is generally consideredto mark a region, not to contain continuous varying information aboutthe object beneath or behind it. In addition, currently no techniqueexists to vary the stencil source data across an object and, therefore,it is uncommon to have an object with multiple stencil values withoutintersecting several stencil operations.

FIG. 5 shows an embodiment of a process 200 for compressing stencilvalues for a tile in accordance with the invention. Stencil valuecompression can advantageously reduce the amount of frame buffer memoryneeded to store stencil values and enable Fast Stencil operations,described below. Graphics-processing systems may or may not implementstencil value compression in conjunction with the above-described Fast Zoperations. In addition, although Fast Stencil operations use theresults produced by stencil value compression, graphics-processingsystems can implement stencil value compression without implementingFast Stencil operations. In the description of the compression process200, reference is also made to FIG. 2.

In brief overview, stencil value compression capitalizes on therecognition that, for many tiles in the frame buffer, the samples withinany given tile may all have the same stencil value. Upon encounteringsuch a tile, the process 200 reduces the stencil values of the samplesin the tile to a single stencil value. This single stencil valuerepresents the stencil value of every sample in the tile, irrespectiveof the number of samples that the tile has. An advantage of theresulting single stencil value is that its size, in terms of used memoryspace, enables reading and writing operations performed on this stencilvalue to occur in a single clock cycle of the graphics processor.

In more specific overview, processor logic determines (step 204), foreach tile in the frame buffer 66, whether the stencil values for allsamples in that tile have the same stencil value. For each tile in whichall stencil values for the tile are the same stencil value, theprocessor logic replaces (step 208) all of the stencil values stored inthe frame buffer 66 with a single stencil value (i.e., the stencil valuethat is common to all samples in the tile). In addition, the processorlogic sets a flag (step 212) to mark that tile as a single-stencil tile(i.e., a tile for which a single stencil value represents the stencilvalue of every sample in the tile). The single stencil value and theflag may be stored together or separately in the frame buffer 66 withthe tile, in the Hi-Z and Hi-S memory 62, or in both.

FIG. 6A and FIG. 6B illustrate an example of saved memory achieved bythe stencil value compression process 200 described in FIG. 5. FIG. 6Ashows an exemplary representation of an 8×8 pixel tile before stencilvalue compression. Here, e.g., each pixel in the tile has eight samples,with each sample having an associated 8-bit stencil value. Accordingly,4096 bits (64 pixels*8 samples/pixel*8-bits/stencil value) of the memory62 hold stencil values for this particular tile. To simplify theillustration, all samples are associated with the same z-plane (z-plane0), which is represented as a 96-bit value. FIG. 6B shows the 8×8 pixeltile after stencil value compression. As shown, the tile has one 8-bitstencil value only, which applies to every sample in the tile. Inaddition, the stencil value compression, in effect, enables a singlememory access (e.g., within a 32-bit cache line) to write or read thesingle stencil value.

Fast Stencil Operation

The ability to represent the stencil values of every sample in a tilewith a single stencil value, in addition to the saving memory andimproving bandwidth, also enables a Fast Stencil operation that canavoid detail rasterization for a tile of pixels. FIG. 7 shows anembodiment of a process 250 for performing Fast Stencil operations inaccordance with the invention. Performance of the Fast Stencil process250 may or may not occur in conjunction with a Fast Z process. Toillustrate that a graphics-processing system can implement the FastStencil process 250 independently of or without implementing Zoperations (e.g., Fast Z), the description refers to the Hi-Z and Hi-Stest module 58 as Hi-S test module 58″ and to the Hi-Z and Hi-S memory62 as Hi-S memory 62″.

At step 254, the tile scan converter 54 receives a primitive from theprimitive assembly 50. The tile scan converter 54 scans through thetiles to identify a tile (step 258) covered fully by the primitive. Atstep 262, the Hi-S test module 58″ tests the fully covered tile, asdescribed above, and determines (step 266) whether the stencil valuesfor all samples of the tile are to undergo the same stencil operation.If the result of the test is the tile will not pass or the outcome isunknown, or if different samples are to undergo different stenciloperations, then the tile undergoes (step 270) detail rasterization (forstencil).

If the tile passes the test, and the Hi-S test module 58″ determinesthat the same stencil operation applies to all stencil values, then theparticular type of stencil operation determines the action taken. Asdescribed above, the particular stencil operation depends upon theresults of the test executed by the Hi-S test module 58″. If, at step272, the particular stencil operation is to replace the existing stencilvalues in the frame buffer 66 for the fully covered tile, then processorlogic replaces (step 274) the old stencil values (or single value) witha single stencil value (e.g., a stencil reference value) associated withthe incoming tile, thereby bypassing detail rasterization.

If, instead, the particular stencil operation is anything other than toreplace the existing stencil values, e.g., increment, decrement, invert,keep, increment_clamp, decrement_clamp, processor logic determines (step278) whether the fully covered tile is marked as a single stencil tile.The processor logic makes this determination by referring to the flagassociated with the tile, whether the flag resides in the Hi-S memory62″ or in the frame buffer 66.

If the tile is a single stencil tile, the processor logic performs (step282) the particular stencil operation on the single stencil value storedin the frame buffer 66 for that tile. For example, if the stenciloperation is to increment, then the processor logic reads the singlestencil value, increments the single stencil value, and writes theincremented stencil value back to the frame buffer 66. This singlemodified stencil value continues to represent each sample in the tile.In addition, the rendering process is again able to bypass detailrasterization.

If the tile in the frame buffer is not a single stencil tile (i.e.,different samples have different stencil values), the processor logic(e.g., a walker engine) walks (step 286) through the stencil values ofthe entire tile, performs the particular stencil operation individuallyon each stencil value, and writes the modified stencil values back tothe frame buffer 66. Although the writing of individual stencil valuesis not as optimized as writing a single stencil value for the wholetile, the ability to bypass detail rasterization provides a measure ofoptimization.

Although described herein predominantly as hardware, embodiments of thedescribed invention may be implemented in hardware (digital or analog),software (program code), or combinations thereof. Program codeimplementations of the present invention may be embodied ascomputer-executable instructions on or in one or more articles ofmanufacture, or in or on computer-readable medium. A computer, computingsystem, or computer system, as used herein, is any programmable machineor device that inputs, processes, and outputs instructions, commands, ordata. In general, any standard or proprietary, programming orinterpretive language can be used to produce the computer-executableinstructions. Examples of such languages include C, C++, Pascal, JAVA,BASIC, Visual Basic, and Visual C++.

Examples of articles of manufacture and computer-readable medium inwhich the computer-executable instructions may be embodied include, butare not limited to, a floppy disk, a hard-disk drive, a CD-ROM, aDVD-ROM, a flash memory card, a USB flash drive, an non-volatile RAM(NVRAM or NOVRAM), a FLASH PROM, an EEPROM, an EPROM, a PROM, a RAM, aROM, a magnetic tape, or any combination thereof. Thecomputer-executable instructions may be stored as, e.g., source code,object code, interpretive code, executable code, or combinationsthereof.

While the invention has been shown and described with reference tospecific preferred embodiments, it should be understood by those skilledin the art that various changes in form and detail may be made thereinwithout departing from the spirit and scope of the invention as definedby the following claims. For example, although described above asseparately operational processes, the Fast Z and Fast Stencil processescan be implemented together in a graphics-processing system.

1. A method of rendering three-dimensional objects, the methodcomprising: partitioning a buffer into tiles, each tile including aplurality of pixels, each pixel of each tile including at least onesample; receiving a primitive associated with an object to be processed;determining that the primitive fully covers one of the tiles of thebuffer; testing a section of the primitive that maps to the fullycovered tile to determine whether that section of the primitive may bedrawn in its entirety; and storing a single-Z plane value in the bufferfor the fully covered tile in response to determining that the sectionof the primitive may be drawn in its entirety, the single-Z plane valueindicating that every sample of the fully covered tile has a depth (Z)value determined by the primitive, and further indicating that thesamples of the fully covered tile are associated with a common z-plane.2. The method of claim 1, wherein the single-Z plane value represents az-plane equation of a plane of the primitive.
 3. The method of claim 2,where the single-Z plane value represents a coordinate location withinthe fully covered tile and a pair of slopes.
 4. The method of claim 1,wherein the step of storing includes the step of replacing, with thesingle-Z plane value, one or more values previously stored in the bufferfor the fully covered tile, each of the previously stored valuesrepresenting a z-plane equation of a previously tested primitive fromwhich at least one sample of the fully covered tile derives its Z value.5. The method of claim 1, wherein the step of testing includes the stepof comparing a range of depth values of the section of primitive mappedto the fully covered tile with minimum and maximum depth values of acorresponding hierarchical tile.
 6. The method of claim 1, wherein theprimitive fully covers every tile of the buffer and each section of theprimitive that maps to one of the fully covered tiles passes a test thatdetermines the corresponding section of the primitive is to be drawn,and further comprising the step of storing the same single z-planeequation, corresponding to a plane of the primitive, for all tiles inthe buffer.
 7. The method of claim 6, further comprising the step ofstoring a single stencil value for all tiles in the buffer.
 8. Agraphics processor comprising: a tile scan converter receiving aprimitive associated with an object to be processed and identifying atile corresponding to the primitive, the tile including a plurality ofpixels, each pixel including at least one sample; a hierarchical Z testmodule determining that the primitive fully covers the tile and that asection of the primitive that maps to the fully covered tile may bedrawn in its entirety; and a buffer, in communication with thehierarchical Z test module, storing a single-Z plane value in responseto a determination by the hierarchical Z test module that the section ofthe primitive may be drawn in its entirety, the single-Z plane valueindicating that every sample of the fully covered tile has a depth (Z)value determined by the primitive, and further indicating that thesamples of the fully covered tile are associated with a common z-plane.9. The graphics processor of claim 8, wherein the single-Z plane valuerepresents a z-plane equation of a plane of the primitive.
 10. Thegraphics processor of claim 9, where the single-Z plane value representsa coordinate location within the fully covered tile and a pair ofslopes.
 11. The graphics processor of claim 8, wherein the storedsingle-Z plane value replaces one or more values previously stored inthe buffer for the fully covered tile, each of the previously storedvalues representing a z-plane equation of a previously tested primitivefrom which at least one sample of the fully covered tile derives its Zvalue.
 12. The graphics processor of claim 8, further comprising a Hi-Zmemory storing hierarchical tiles, each hierarchical tile correspondingto one of the tiles, and wherein the Hi-Z test module compares a rangeof depth values of the section of primitive mapped to the fully coveredtile with one or more depth values of the hierarchical tilecorresponding to fully covered tile.
 13. A video graphics systemcomprising: a graphics processor including: a tile scan converterreceiving a primitive associated with an object to be processed andidentifying a tile corresponding to the primitive, the tile including aplurality of pixels, each pixel including at least one sample; ahierarchical Z test module determining that the primitive fully coversthe tile and that a section of the primitive that maps to the fullycovered tile may be drawn in its entirety; and a buffer, incommunication with the hierarchical Z module, storing a single-Z planevalue in response to a determination by the hierarchical Z test modulethat the section of the primitive may be drawn in its entirety, thesingle-Z plane value indicating that every sample of the fully coveredtile has a depth (Z) value determined by the primitive, and furtherindicating that the samples of the fully covered tile are associatedwith a common z-plane.
 14. The video graphics system of claim 13,wherein the single-Z plane value represents a z-plane equation of aplane of the primitive.
 15. The video graphics system of claim 14, wherethe single-Z plane value represents a coordinate location within thefully covered tile and a pair of slopes.
 16. The video graphics systemof claim 13, wherein the stored single-Z plane value replaces one ormore values previously stored in the buffer for the fully covered tile,each of the previously stored values representing a z-plane equation ofa previously tested primitive from which at least one sample of thefully covered tile derives its Z value.
 17. The video graphics system ofclaim 13, further comprising a Hi-Z memory storing hierarchical tiles,each hierarchical tile corresponding to one of the tiles, and whereinthe Hi-Z test module compares a range of depth values associated withthe section of the primitive mapped to the fully covered tile with oneor more depth values of the hierarchical tile corresponding to the fullycovered tile.
 18. The video graphics system of claim 13, furthercomprising a display in communication with the graphics processor.